Preparation of caprolactam

ABSTRACT

A process for the preparation of caprolactam is provided, wherein 
     a) a mixture (I) containing 6-aminocapronitrile and water is reacted in the gas phase, in the presence of a catalyst, to give a mixture (II) containing caprolactam, ammonia, water, high-boiling components and low-boiling components, 
     b) ammonia is then removed from the mixture (II) to give a mixture (III) containing caprolactam, water, high-boiling components and low-boiling components, 
     c) water is then removed from the mixture (III) to give a mixture (IV) containing caprolactam, high-boiling components and low-boiling components, and 
     d) a solid (V) containing caprolactam is then obtained from the mixture (IV) by crystallization, the proportion by weight of caprolactam in the solid (V) being greater than in the mixture (IV).

The present invention relates to a process for the preparation ofcaprolactam, wherein

a) a mixture (I) containing 6-aminocapronitrile (“ACN”) and water isreacted in the gas phase, in the presence of a catalyst, to give amixture (II) containing caprolactam, ammonia, water, high-boilingcomponents and low-boiling components,

b) ammonia is then removed from the mixture (II) to give a mixture (III)containing caprolactam, water, high-boiling components and low-boilingcomponents,

c) water is then removed from the mixture (III) to give a mixture (IV)containing caprolactam, high-boiling components and low-boilingcomponents, and

d) a solid (V) containing caprolactam is then obtained from the mixture(IV) by crystallization, the proportion by weight of caprolactam in thesolid (V) being greater than in the mixture (IV).

Processes for the preparation of caprolactam are generally known.

It is also generally known, for example from Ullmann's Encyclopedia ofIndustrial Chemistry, 5th Ed., Vol. A5, VCH Verlagsgesellschaft mbH,Weinheim (Germany), 1986, pages 46-48, or Kirk-Othmer, Encyclopedia ofChemical Technology, 4th Ed., Vol. 4, John Wiley & Sons, New York, 1992,page 836, that caprolactam used for the preparation of polymers musthave a purity of 99.9 to 99.94%, the main impurity conventionally beingwater in an amount of 0.04 to 0.1%. Other impurities must only bepresent in an amount of at most a few ppm.

Thus caprolactam can be prepared by a Beckmann rearrangement ofcyclohexanone oxime with sulfuric acid or oleum. After neutralization ofthe resulting mixture with ammonia, the caprolactam can be obtained fromthe ammonium sulfate formed as a by-product by extraction with anorganic solvent.

Depending on the processes for the preparation of the educts used toprepare the cyclohexanone oxime, such as cyclohexanone andhydroxylammonium sulfate, and on the oximation and rearrangementconditions, the crude caprolactam obtained by a Beckmann rearrangementcontains different types and amounts of impurities. Typical impuritiesin crude caprolactam prepared by a Beckmann rearrangement areC-methylcaprolactams, 6-methylvalerolactam and n-pentylacetamide.

Various processes are described for the purification of crudecaprolactam obtained by a Beckmann rearrangement.

According to DE-A-1253716, the crude caprolactam can be purified byhydrogenation in suspension, in the presence of a catalyst and with theaddition of an acid.

According to DE-A-1253716, the crude caprolactam can be purified byhydrogenation in suspension, in the presence of a catalyst and with theaddition of a base.

DD-A-75083 describes a process for the purification of crude caprolactamin which the crude caprolactam is first distilled and then dissolved inan organic solvent, hydrogenated in the presence of a catalyst and thentreated with an ion exchanger.

According to EP-A-411455, the important characteristic quality featuresof caprolactam can be preserved by hydrogenating the crude caprolactamcontinuously in a liquid phase process.

Crude caprolactam obtained by the hydroformylation of 3-pentenoic acidand/or its esters to give 5-formylvaleric acid (esters) as main productsand 4- and 3-formylvaleric acid (esters) as by-products, separation ofthis (these) branched formylvaleric acid (esters) by extraction (WO97/02228) or distillation (WO 97/06126), aminating hydrogenation of5-formylvaleric acid (esters) to 6-aminocaproic acid (esters) and/or6-aminocaproic acid amide, and cyclization of 6-aminocaproic acid(esters) or 6-aminocaproic acid amide, contains other typicalimpurities.

Thus it is known e.g. from WO 99/48867, Example 1, to crystallize crudecaprolactam obtained from 5-formylvaleric acid esters, according to WO98/37063, Example 9, from mixtures of 6-aminocaproic acid,6-aminocaproic acid amide and corresponding oligomers, by the additionof 10% by weight of water. This crude caprolactam, from whichhigh-boiling and low-boiling components were not separated beforecrystallization, contained 6345 ppm of N-methylcaprolactam, 100 ppm of5-methylvalerolactam, 78 ppm of valeramide and other impurities. Thecrude caprolactam/water melt was homogenized at 50° C. and then cooledto 30° C. The crystals which precipitated out were filtered off andwashed 2 to 3 times with aqueous caprolactam. The 5-methylvalerolactamand valeramide contents were reduced to 1 ppm and theN-methylcaprolactam content to 51 ppm. 33.7 g of pure lactam wereobtained from 73.6 g of crude lactam (caprolactam yield: 45.8%). Thecharacteristic of the volatile bases (VB) was only achieved by a secondcrystallization. If high-boiling and low-boiling components wereseparated from the crude caprolactam before crystallization, accordingto WO 99/48867, Example 3, the caprolactam yield after crystallizationwas 52%.

It is further known from WO 99/65873 selectively to adsorb caprolactamfrom mixtures with 4-ethyl-2-pyrrolidone, 5-methyl-2-piperidone,3-ethyl-2-pyrrolidone and 3-methyl-2-piperidone or octahydrophenazine onadsorbents like activated carbon, molecular sieves or zeolites to givehighly pure caprolactam after desorption. This separation of caprolactamcan be followed by crystallization from the melt or crystallization froma solvent.

It is further known to purify, by crystallization, crude caprolactamwhich, starting from 6-aminocapronitrile, is first hydrolyzed with waterto 6-aminocaproic acid, according to WO 98/37063, claim 8. Water andammonia formed by hydrolysis are then separated off, the 6-aminocaproicacid formed is cyclized and the crude caprolactam obtained iscrystallized according to WO 99/48867.

Caprolactam can also be obtained by reacting ACN with water in theliquid phase, in the presence or absence of a catalyst, with the releaseof ammonia.

In addition to caprolactam, water, ammonia and optionally another liquiddiluent, the mixture obtained in this reaction contains impuritiesboiling above caprolactam (“high-boiling components”) and impuritiesboiling below caprolactam (“low-boiling components”).

It is known from the Example in U.S. Pat. No. 496,941 that, after theseparation of water, solvent, ammonia, low-boiling component andhigh-boiling component from a mixture obtained by reacting ACN withwater and solvent, a crude caprolactam is obtained with a purity of99.5%.

Other methods of purification are described for a crude caprolactamobtained from ACN in the liquid phase since the impurities in this typeof crude caprolactam is [sic] markedly different from those in a crudecaprolactam obtained by other processes, as described in U.S. Pat. No.5,496,941.

In a first step, according to U.S. Pat. No. 5,496,941, ACN is convertedto caprolactam in the liquid phase, low-boiling components, water,ammonia and optionally other solvents are simultaneously separated off,high-boiling components are separated off to give a crude caprolactamwith a purity of 99.5%, this crude caprolactam is hydrogenated in thepresence of a catalyst, the product obtained is treated with an acidicion exchanger or sulfuric acid and the resulting product is distilled inthe presence of a base.

WO 96/20923 discloses a method of purifying crude caprolactamoriginating from the liquid phase cyclization of 6-aminocapronitrilewith water in the presence of a solvent and heterogeneous catalysts. Inthis case, crude caprolactam is first hydrogenated, then treated withacidic agents and finally distilled in the presence of alkali. Thedisadvantage of this method of purification is that three separatereaction steps are required to prepare pure caprolactam.

The cyclization of 6-aminocapronitrile in the gas phase in the presenceof water and a catalyst, for example as described in EP-A-659 741, WO96/22974, DE 19632006, WO 99/47500 or WO 99/28296, gives a crudecaprolactam in which the typical impurities are different from those ina crude caprolactam obtained by another process. Examples of typicalimpurities in a crude caprolactam obtained from ACN in the gas phase arecyanoalkyl- and aminoalkyl-substituted caprolactam derivatives andtetrahydroazepine derivatives, such as N-cyanopentylhexamethyleneimine,N-cyanopentylcaprolactam and N-aminohexylcaprolactam. In purecaprolactam, these impurities contribute to a degradation of the qualitycharacteristics generally known for caprolactam, for example fromUllmann's Encyclopedia of Industrial Chemistry, 5th Ed., Vol. A5, VCHVerlagsgesellschaft mbH, Weinheim (Germany), 1986, pages 46-48, orKirk-Othmer, Encyclopedia of Chemical Technology, 4th Ed., Vol. 4, JohnWiley & Sons, New York, 1992, page 836, such as the values for the freeand volatile base [sic] and UV characteristics.

It is an object of the present invention to provide a process whichmakes it possible to prepare, in high purity and in a technically simpleand energy-saving manner, caprolactam which has been obtained from ACNin the gas phase.

We have found that this object is achieved by the process defined at theoutset.

In step a), a mixture (I) containing 6-aminocapronitrile, water andoptionally liquid diluent is converted in the gas phase, in the presenceof a solid which promotes the reaction catalytically, to a mixture (II)containing caprolactam, ammonia, water, optionally liquid diluent,high-boiling components and low-boiling components.

The ACN required for step a) can be obtained from adipodinitrile, as isgenerally known from Ullmann's Encyclopedia of Industrial Chemistry, 5thEd., Vol. A5, VCH Verlagsgesellschaft mbH, Weinheim (Germany), 1986,page 46, FIG. 8.

Particularly appropriate here is the partial catalytic hydrogenation ofadipodinitrile in the presence of ammonia as solvent and e.g. in thepresence of rhodium on magnesium oxide (U.S. Pat. No. 4,601,859), Raneynickel (U.S. Pat. No. 2,762,835, WO 92/21650) or nickel on aluminumoxide (U.S. Pat. No. 2,208,598) as a suspension catalyst or Cu—Co—Znspinel (DE-B-954416, U.S. Pat. No. 2,257,814) or iron (DE-A-42 35 466)as a fixed bed catalyst, or a process according to U.S. Pat. No.2,245,129, U.S. Pat. No. 2,301,964, EP-A-150295 or FR-A-2 029 540, or aprocess described in U.S. Pat. No. 5,496,941.

The adipodinitrile required for this reaction is prepared industrially,e.g. by the double hydrocyanation of butadiene in the presence ofnickel-containing catalysts, and is commercially available, e.g. fromAldrich-Chemie Gesellschaft mbH & Co. KG, Steinheim, Germany. Theconversion of the mixture (I) to the mixture (II) can be carried outaccording to EP-A-659 741, WO 96/22974, DE 19632006, WO 99/47500 or WO99/28296 for example.

The reaction can preferably be carried out in the gas phase attemperatures generally of 200 to 550° C., preferably of 250 to 400° C.;the pressure ranges generally from 0.01 to 10 bar and is preferablyatmospheric pressure, it being necessary to ensure that the reactionmixture is predominantly gaseous under the conditions used.

The catalyst loads are usually 0.05 to 2, preferably 0.1 to 1.5 andparticularly 0.2 to 1 kg of 6-aminocapronitrile per liter of catalystvolume per hour.

The reaction can be carried out batchwise or, preferably, continuously.

Suitable reactors are advantageously those which are generally known forgas phase reactions on moving or stationary solid catalysts. It ispreferred to use a fluidized bed reactor or, preferably, a fixed bedreactor such as a tray reactor, especially a tubular reactor.Combinations of such reactors are also possible.

The amount of water used is generally 1 to 50, preferably 1 to 10 molper mol of ACN.

The mixture (I) can also contain other organic compounds which aregaseous under the reaction conditions, such as alcohols, amines oraromatic or aliphatic hydrocarbons.

Examples of suitable catalytically active compounds in the catalysts aresilicon dioxide in the form of pyrogenic silicon dioxide, silica gel,kieselguhr, quartz or mixtures thereof, copper chromite, preferablyaluminum oxide, titanium oxide, preferably titanium dioxide, lanthanumphosphates and lanthanum oxides, as well as mixtures of such compounds.

Aluminum oxide is suitable in any modifications which can be obtained byheating the aluminum hydroxide precursor compounds (gibbsite, boehmite,pseudoboehmite, bayerite and diaspore) at varying temperatures. Theseinclude especially gamma- and alpha-aluminum oxide and mixtures thereof.

Titanium dioxide is amorphous and is suitable in any of itsmodifications, preferably anatase and rutile, and mixtures of suchmodifications.

Lanthanum phosphates are suitable, individually or in a mixture, intheir various modifications, stoichiometric ratios of lanthanum tophosphate unit and degrees of condensation of the phosphate units(monophosphate, oligophosphates such as diphosphates or triphosphates,polyphosphates).

These compounds can be used in the form of powders, chips, grit, strandsor tablets (produced by compression). The form of the compounds normallydepends on the requirements of the particular reaction procedure, powderor chips advantageously being used in the fluidized bed procedure. Inthe fixed bed procedure, it is conventional to use tablets or strandswith diameters of between 1 mm and 6 mm.

The compounds can be used in the pure form (content of the individualcompounds >80% by weight), as a mixture of the abovementioned compounds,in which case the sum of the abovementioned compounds should be >80% byweight, or as a supported catalyst, in which case the abovementionedcompounds can be applied to a mechanically and chemically stablesupport, usually with a high surface area.

The pure compounds may have been prepared by precipitation from aqueoussolutions, e.g. titanium dioxide by the sulfate process, or by otherprocesses such as the pyrogenic preparation of fine aluminum oxide,titanium dioxide or zirconium dioxide powders, which are commerciallyavailable.

A choice of several methods is available for the preparation of mixturesof the different compounds. The compounds, or their precursor compoundswhich can be converted to the oxides by calcination, can be preparede.g. by joint precipitation from solution. A very good distribution ofthe two compounds used is generally obtained by this method. Thecompound or precursor mixtures can also be obtained by precipitation ofone compound or precursor in the presence of the second compound orprecursor present as a suspension of fine particles. Another methodconsists in mechanically mixing the compound or precursor powders, itbeing possible for this mixture to be used as a starting material forthe production of strands or tablets.

In principle, supported catalysts can be prepared by any of the methodsdescribed in the literature. Thus the compounds can be applied to thesupport in the form of their sols simply by impregnation. The volatileconstituents of the sol are conventionally removed from the catalyst bydrying and calcination. Such sols are commercially available fortitanium dioxide and aluminum oxide.

Another possible way of applying layers of the catalytically activecompounds consists in hydrolyzing or pyrolyzing organic or inorganiccompounds. Thus a ceramic support can be coated with a thin layer oftitanium dioxide by hydrolyzing titanium isopropylate or other Tialkoxides. Other suitable compounds are TiCl₄ and aluminum nitrate,inter alia. Suitable supports are powders, strands or tablets of saidcompounds themselves or of other stable compounds like steatite orsilicon carbide. The supports used can be in a macroporous form in orderto improve the material transport.

The reaction can be carried out in the presence of a gas which is inertas regards the conversion of the mixture (I) to the mixture (II),preferably argon and in particular nitrogen. The volume ratio of theinert gas to the ACN, which is gaseous under reaction conditions, canadvantageously be up to 100.

In step b), ammonia is removed from the mixture (II) to give a mixture(III) containing caprolactam, water, optionally liquid diluent,high-boiling components and low-boiling components.

In principle, the separation of the ammonia from the mixture (II) can beeffected by methods known per se for the separation of materials, suchas extraction or, preferably, distillation, or a combination of suchmethods.

The distillation can advantageously be carried out at bottomtemperatures of 60 to 220° C., especially of 100 to 220° C. Thepressure, measured at the top of the distillation performance [sic], isconventionally set at 2 to 30 bar absolute.

Suitable apparatuses are those conventionally used for distillation, forexample the ones described in Kirk-Othmer, Encyclopedia of ChemicalTechnology, 3rd Ed., Vol. 7, John Wiley & Sons, New York, 1979, pages870-881, such as sieve-plate columns, bubble-cap columns or packedcolumns.

The distillation can be carried out in several columns, such as 2 or 3,but advantageously in a single column.

In step c), water and optionally liquid diluents are removed from themixture (III) to give a mixture (IV) containing caprolactam,high-boiling components and low-boiling components.

If a liquid diluent has been used in step a), water and liquid diluentcan be separated off simultaneously in step c) or the water can beseparated off before or after the liquid diluent.

In principle, the water can be separated from the mixture (III) bymethods known per se for the separation of materials, such asextraction, crystallization or, preferably, distillation, or acombination of such methods.

The distillation can advantageously be carried out at bottomtemperatures of 50 to 250° C., especially of 100 to 230° C.

Suitable apparatuses are those conventionally used for distillation, forexample the ones described in Kirk-Othmer, Encyclopedia of ChemicalTechnology, 3rd Ed., Vol. 7, John Wiley & Sons, New York, 1979, pages870-881, such as sieve-plate columns, bubble-cap columns or packedcolumns.

The distillation can be carried out in several columns, such as 2 or 3,but advantageously in a single column.

A heat-coupled multistage separation of the water and optionally theliquid diluent is particularly preferred.

Before the mixture (IV) is introduced into step d), it is appropriate toseparate off the low-boiling component [sic] and high-boiling component[sic], advantageously only the high-boiling components, especiallyneither the low-boiling component [sic] nor the high-boiling component[sic] and particularly advantageously only the low-boiling componentsfrom the mixture (IV).

If the low-boiling components and high-boiling components are separatedfrom the mixture, the low-boiling components can be separated offbefore, after or together with the high-boiling components.

In the case where the low-boiling component [sic] and high-boilingcomponent [sic], or only the high-boiling component [sic], or only thelow-boiling component [sic], are separated off, the separation can beeffected in principle by methods known per se for the separation ofmaterials, such as extraction, crystallization or, preferably,distillation, or a combination of such methods.

The distillation can advantageously be carried out at bottomtemperatures of 50 to 250° C., especially of 100 to 230° C. Thepressure, measured at the top of the distillation performance [sic], isconventionally set at 1 to 500 mbar absolute, preferably 5 to 100 mbarabsolute.

Suitable apparatuses are those conventionally used for distillation, forexample the ones described in Kirk-Othmer, Encyclopedia of ChemicalTechnology, 3rd Ed., Vol. 7, John Wiley & Sons, New York, 1979, pages870-881, such as sieve-plate columns, bubble-cap columns or packedcolumns.

The distillation for separating off the low-boiling components can becarried out in several columns, such as 2 or 3, but advantageously in asingle column.

The distillation for separating off the high-boiling components can becarried out in several columns, such as 2 or 3, but advantageously in asingle column.

In step d), a solid (V) containing caprolactam is obtained from themixture (IV) by partial crystallization, the proportion by weight ofcaprolactam in the solid (V) being greater than in the mixture (IV).

The sum of the contents of high-boiling and low-boiling components, notincluding water and organic diluents, in the mixture (IV) used in stepd) is advantageously at least 100 ppm by weight, preferably 200 ppm byweight, particularly preferably at least 500 ppm by weight andespecially at least 1000 ppm by weight, based on the mixture (IV).

The crystallization can be effected batchwise or continuously.

The crystallization can be effected with the addition of an aid such asan organic or inorganic liquid diluent, for example water, butpreferably without the addition of an aid.

The crystallization can be effected in one or more stages, such as two,three or four stages, preferably one stage. In another preferredembodiment of the invention, the crystallization can be effected asfractional crystallization.

In the case of fractional crystallization, all the stages producing acrystalline product (caprolactam) which is purer than the initial crudeproduct (crude caprolactam) are conventionally called purificationstages, and all the other stages are conventionally called refiningstages. It is advisable here to operate multistage processes accordingto the countercurrent principle, whereby, after the crystallization ineach stage, the crystalline product is separated from the remainingliquid phase (“mother liquor”) and transferred to the appropriate stagewith the next highest degree of purity, the crystallization residuebeing transferred to the appropriate stage with the next lowest degreeof purity.

Advantageously, the temperature of the solution or melt duringcrystallization is not higher than the melting point of caprolactam (70°C.) and is preferably between −10 [sic] and the melting point ofcaprolactam and especially between 20 [sic] and the melting point ofcaprolactam. The solids content in the crystallizer is conventionallybetween 0 and 70 g, preferably between 30 and 60 g, per 100 g of charge.

In another advantageous embodiment of the invention, the crystallizationis effected in apparatuses in which the crystals grow on cooled surfacesin the crystallization apparatus, i.e. are fixed in the apparatus (e.g.layer crystallization process from Sulzer Chemtech (Switzerland) orstatic crystallization process from BEFS PROKEM (France)).

The crystallization can also be effected by cooling apparatus walls orby evaporating a solution of the crude caprolactam under reducedpressure. Five to 30% by weight solutions of crude caprolactam in aliquid diluent, especially water, are particularly suitable for thispurpose.

In the case of crystallization by cooling, the heat can be removed viascraped wall chillers connected to a stirred tank or an unstirredvessel. The crystal suspension can be circulated by means of a pump. Afurther possibility is to remove the heat via the wall of a tank with awall-fitted stirrer. Another preferred embodiment of crystallization bycooling is the use of cooling disk crystallizers, e.g. thosemanufactured by Gouda (Holland). In another suitable variant ofcrystallization by cooling, the heat can be removed via conventionalheat exchangers (preferably shell-and-tube or parallel-plate heatexchangers). In contrast to scraped wall chillers, tanks withwall-fitted stirrers or cooling disk crystallizers, these apparatuses donot possess a device for preventing layers of crystals from forming onthe heat-transfer surfaces. If, during operation, a situation is reachedwhere the resistance to heat transition due to the formation of layersof crystals becomes excessive, the conventional procedure is to switchover to a second apparatus. During the operating period of the secondapparatus, the first apparatus can be regenerated (preferably by meltingthe layer of crystals or flushing the apparatus with unsaturatedsolution). If the resistance to heat transition in the second apparatusbecomes excessive, the procedure is to switch back to the firstapparatus, and so on. This variant can also be operated with more thantwo apparatuses in alternation. The crystallization can also be effectedby conventional evaporation of the solution under reduced pressure.

The solid-liquid separation methods known per se are suitable forseparating the mother liquor from the caprolactam which has crystallizedout.

In one preferred embodiment of the invention, the crystals can beseparated from the mother liquor by filtration and/or centrifugation.Advantageously, the filtration or centrifugation can be preceded bypreliminary concentration of the suspension, for example by means of oneor more hydrocyclones. Centrifuges known per se, which operate batchwiseor continuously, are suitable for the centrifugation. It is mostadvantageous to use pusher centrifuges, which can be operated in one ormore stages. Screen-conveyor centrifuges or helical-conveyor centrifuges(decanters) are also suitable. The filtration can advantageously beeffected by means of suction filters, which can be operated batchwise orcontinuously and with or without a stirrer, or by means of belt filters.The filtration can generally be carried out under superatmosphericpressure or under reduced pressure.

During and/or after the solid-liquid separation, provision can be madefor further process steps to increase the purity of the crystals orcrystal cake. In one particularly advantageous embodiment of theinvention, the separation of the crystals from the mother liquor isfollowed by washing and/or sweating of the crystals or crystal cake inone or more stages.

In the case of washing, the amount of washing liquor should preferablybe between 0 and 500 g per 100 g of crystalline product, preferablybetween 30 and 200 g per 100 g of crystalline product.

Suitable washing liquors are organic or inorganic liquids or mixtures ofsuch liquids, examples of preferred washing liquors being

a) in the case where a liquid diluent has been used in thecrystallization of step d), said liquid diluent,

b) a melt of a crystalline product obtained in a crystallization stageof step d),

c) a mother liquor obtained in a crystallization stage of step d), or

d) a melt of an educt used in a crystallization stage of step d).

The washing can be effected in apparatuses conventionally used for thispurpose. It is advantageous to use wash columns, in which the separationof the mother liquor and the washing take place in one apparatus,centrifuges, which can be operated in one or more stages, or suctionfilters or belt filters. The washing can be effected on centrifuges orbelt filters in one or more stages, it being possible for the washingliquor to be conveyed in countercurrent to the crystal cake.

Particularly in the case of crystallization without the addition of anaid, the washing liquor can be recycled into the crystallization,optionally after impurities have been separated off.

Sweating is conventionally understood as meaning a local melting ofcontaminated regions. The amount of sweating should advantageously be0.1 to 90 g of melted crystalline product per 100 g of crystallineproduct prior to sweating, preferably 5 to 35 g of melted crystallineproduct per 100 g of crystalline product. It is particularly preferredto carry out the sweating on centrifuges or belt filters. It may also beappropriate to combine washing and sweating in one apparatus.

Particularly in the case of crystallization without the addition of anaid, the mother liquor can be recycled into the crystallization,optionally after impurities have been separated off.

Caprolactam can be obtained in a purity of at least 99.90% by weight,preferably 99.90 to 99.99% by weight, by the present process.

The caprolactam obtainable by the process according to the invention canbe used for the preparation of polyamides like polycaprolactam.

We claim:
 1. A process for the preparation of caprolactam, wherein a) amixture (I) containing 6-aminocapronitrile and water is reacted in thegas phase, in the presence of a catalyst, to give a mixture (II)containing caprolactam, ammonia, water, high-boiling components andlow-boiling components, b) ammonia is then removed from the mixture (II)to give a mixture (III) containing caprolactam, water, high-boilingcomponents and low-boiling components, c) water is then removed from themixture (III) to give a mixture (IV) containing caprolactam,high-boiling components and low-boiling components, and d) a solid (V)containing caprolactam is then obtained from the mixture (IV) bycrystallization, the proportion by weight of caprolactam in the solid(V) being greater than in the mixture (IV).
 2. A process as claimed inclaim 1 wherein the mixture (I) additionally contains an inert gaseousdiluent.
 3. A process as claimed in claim 2 wherein titanium dioxide,aluminum oxide or lanthanum phosphate is used in step a) as thecatalytically active component of the catalyst.
 4. A process as definedin claim 1, wherein the low boilers are separated between steps c) andd).
 5. A process as defined in claim 1, wherein the high boilers areseparated between steps c) and d).
 6. A process as defined in claim 1,wherein the low boilers and high boilers are separated between steps c)and d).
 7. A process as defined in claim 1, wherein the sum of thecontents of high boilers and low boilers in mixture (IV) used in stepd), calculated without water and any diluent still present, is at least100 ppm by weight.
 8. A process as defined in claim 1, whereincrystallization in step d) is carried out in the absence of auxiliaryagents.
 9. A process as defined in claim 1, wherein crystallization instep d) is carried out on a cooled surface, on which solid (V) grows.10. A process as defined in claim 1, wherein the mother liquor obtainedafter crystallization in step d) is mixed with mixture (IV) and recycledto step d).
 11. A process as defined in claim 1, wherein crystallizationin step d) is carried out batchwise.